1. Field of the Invention
The invention relates to the field of high voltage circuit interruption in electrical devices such as switchgears, transformers, and the like, and in particular concerns high voltage current limiting fuses or expulsion fuses, circuit breakers, circuit interrupters, separable cable connectors, or the like, comprising an arc-quenching composition which is adapted to rapidly evolve a gas in the presence of an electric arc to aid in arc extinction, and thereby quickly and effectively break the circuit. More particularly, the invention is directed to arc-quenching coating compositions having excellent arc-quenching properties and improved track resistance properties that are relatively easy to apply and operationally position in high voltage current limiting fuses.
2. Prior Art
Expulsion fuses or gas-evolving fuses have been used extensively for high voltage circuit interruption in switchgears, transformers, and other electrical equipment. It is generally known that the use of arc-quenching or gas-evolving materials in such a circuit interruption device positioned in contact with the fusible element aids in, inter alia., deionizing, cooling, and thus quenching of the electric arc created under fault current conditions.
A typical high voltage fuse comprises a generally tubular casing of electrical insulating material; a pair of terminal elements closing each of the opposite ends of the casing; a pulverulent arc-quenching filler material of high dielectric strength inside the casing such as sand, mica beads, or finely divided quartz; a fusible element or elements made of a highly conductive material such as silver submersed in the filler and conductively interconnecting the terminal elements, the fusible elements typically being wound in a parallel-connected relationship along the length of a supporting core; a core of high dielectric strength electrically insulating high temperature material such as ceramic, the core providing support for the fusible element by longitudinally and radially extending, i.e., providing fins having a cross-shaped, star-shaped or the like cross-section, along the longitudinal axis of the casing; and a gas-evolving material distributed along the length of the core or comprising part of the core itself in contact with the fusible element or elements.
In operation, when the high voltage current limiting fuse is subjected to an applied current that exceeds the current carrying capability of the fusible element, the excessive current generates heat whereby the fusible element attains a fusion temperature which initiates melting and vaporization of the fusible element. Electrical arcing thereby occurs as the fusible element or metal vapors rapidly expand to many times the volume originally occupied by the fusible element. These vapors, therefore, expand into the space between the filler material where they condense through heat transfer into the filler and are no longer available for current conduction. In addition, the gas-evolving material distributed along the length of the core or comprising part of the core is adapted to rapidly evolve a gas during arcing and thereby produce a deionizing action and a cooling effect on the arc, which facilitates arc extinction and also reduces the occurrence of restriking and tracking, i.e., fuse conduction after the interruption of the overload current.
A good arc-extinguishing material must be capable of rapidly generating a large volume of non-combustible and non-toxic gas within a short time after the arc has been struck. The arc-extinguishing material and its solid residue in a fused state must be relatively non-conductive so as to prevent restriking or tracking of the arc by conductance through the fused compound, thereby avoiding re-establishing a current flow through the material after interruption. In addition, the arc-extinguishing material must be relatively insoluble in water so that it will not be affected by water present in the atmosphere. Furthermore, the arc-extinguishing material should be moldable or positionable into a self-sustaining structure without large mounts of inert binder.
The art has been focused on formulating arc-quenching compositions which are easily molded into strong self-sustaining structures and then installed in a fuse or circuit interruption device. For example, U.S. Pat. No. 4,339,742--Leach et al. disclose a high voltage fuse having a plurality of block-shaped gas-evolving members attached to a plurality of fuse elements wound about a supporting core. The structural gas-evolving members are fabricated with narrow slits to easily mount to the fuse elements at desired locations.
U.S. Pat. No. 4,166,266--Kozacka et al. disclose an electric fuse having a core for supporting the fusible elements made of a longitudinally extending structural gas-evolving rod. U.S. Pat. No. 4,625,195--Robbins discloses an electric fuse having positioning means on the core to engage a gas-evolving structural member having a lateral protrusion integrally formed on the surface of the gas-evolving member. See, for example, U.S. Pat. Nos. 3,582,586; 3,761,669; 4,251,649; 4,340,790; and, 4,444,671 for more structural applications of arc-quenching or gas-evolving compositions.
However, these arc-quenching self-sustaining strong structural materials suffer from disadvantages. The conventional self-sustaining arc-quenching materials having high physical strength comprise a gas-evolving material combined with a thermoplastic or thermosetting polymeric binder. The binder compositions, although providing physical strength and moldability to the generally weak arc-quenching materials to form self-sustaining arc-quenching structural materials, are generally highly carbonizing materials. Therefore, upon arcing conditions, the binder decomposes and forms conductive carbon residues in the circuit interruption device which thereby causes undesirable tracking and restriking of the arc.
As appreciated in the art, typical arc-quenching materials alone are structurally complex, difficult to manufacture into satisfactory structural shapes, and, therefore, cannot be effectively installed in a high voltage current limiting device without this expensive structural modification of combining the arc-quenching material with a structural polymeric binder. The resulting carbonizing properties of the polymeric binder has been tolerated as an unavoidable by-product in order to improve the moldability and physical strength of the arc-quenching material.
Melamine and melamine derived nitrogen-containing compounds were first disclosed as effective arc-extinguishing materials in U.S. Pat. No. 2,526,448--Amundson et al. Melamine is a heterocyclic nitrogen compound containing a 1,3,5-triazine gas-evolving group. Melamine is a white crystalline powder having a melting point of about 345.degree. C. and sublimes, i.e. its solid transforms directly to vapor without passing through its liquid phase, at its melting temperatures and below. Melamine has the following general chemical structure: ##STR1##
However, melamine and melamine derived nitrogen-containing compounds although having excellent arc-extinguishing abilities, have been discovered to be incapable of being fabricated, i.e., molded, extruded, etc., into satisfactory structural shapes and further lacked effectiveness at lower power conditions. Therefore, it became necessary in the art, as discussed generally above, to provide melamine in combination with a suitable organic binder in order to provide sufficient moldability and physical strength to the arc-extinguishing materials, such as improved tensile strength, percent elongation and the amount of energy required to rupture the product. The binder also provided lower power circuit interruption.
U.S. Pat. No. 3,582,586--Jones discloses an arc-interrupting composition comprising melamine and a thermoplastic organic polymeric binder which provides improved structural properties of the arc-quenching materials and effectiveness to arcing at lower amperage circuit interruption conditions, below which melamine was effective. Jones discloses that effective binders are thermoplastic resins including polyethylene, polypropylene, polytetrafluoroethylene, acrylic and acetal resins. Jones further discloses that another binder may be thermosetting resins including melamineformaldehyde resins.
These thermoplastic and thermosetting polymeric binders have been found useful generally in arc-interrupting compositions based upon melamine or related compounds because these binders volatilize in the presence of an electric arc at lower power conditions than necessary to sublime melamine which thereby produces large volumes of gas to drive the melamine into the core of the are and to extinguish the arc. In addition, the binders provide compositions with good molding and forming ability, stability and electrical insulating properties and physical strength.
However, the organic structural binders suffer from the disadvantage that they readily carbonize in air under arcing conditions. The arc-quenching compositions containing the organic structural binders typically have a high carbon content which therefore decomposes under arcing conditions to produce carbon residues. The carbon residues are conductive and therefore cause tracking of the are and create difficulties in quenching the arc. Furthermore, the compositions with binders are typically expensive to formulate and fabricate into the desired structural shapes for placement in the circuit interruption device. The binder must first be mechanically homogenized with the arc-quenching material by using plastic compounding energy consumptive techniques such as milling or the like, and then modified into desired shapes by using plastic processing techniques, such as injection/compression molding, extrusion, pultrusion and the like. Furthermore the mixing of the binder and the arc-quenching material may not provide optimal distribution of the arc-quenching material.
U.S. Pat. No. 3,761,660--Jones discloses an arc-interrupting composition having improved anti-tracking properties comprising melamine, hydrated alumina, and a thermoplastic organic binder. Jones discloses that the addition of hydrated alumina, Al.sub.2 O.sub.3.3H.sub.2 O provides non carbonizing properties to the arc-quenching composition which is attributed to its release of water of hydration for effective arc-quenching and to its catalyzing the oxidation of carbonaceous material to thereby cause a clean bum and prevent carbon deposits or residues on the arc exposure surfaces. Thus, the hydrated alumina reduces the tendency of the organic binder upon arcing conditions to carbonize on the surface of material and form a conductive path for are tracking. However, the use of hydrated materials in fuses leads to possible corrosion damage to the fuse components from the evolved water of hydration during arcing conditions.
Other examples of arc-quenching compositions comprising arc-quenching materials and organic binders are disclosed in the following publications. U.S. Pat. No. 4,251,699--Wiltgen, Jr. discloses another arc-quenching composition comprising dicyandiamide. Wiltgen, Jr. discloses that the dicyandiamide composition is typically provided in combination with an organic binder. However, dicyandiamide has a 210.degree. C. melting point, lower than the melting point of melamine, and sublimes at its melting point and below. Therefore, dicyandiamide has a lower thermal stability than melamine and therefore tends to disassociate and evolve gases at lower than desirable conditions. Furthermore, the dicyandiamide contains a reactive cyano group in the molecule which produces toxic gas upon decomposition under arcing conditions. U.S. Pat. No. 4,444,671--Wiltgen, Jr. discloses an arc-extinguishing material comprising hexamethylenetetramine and binder.
U.S. Pat. No. 4,975,551--Syvertson discloses an arc-extinguishing composition comprising effective amounts by weight of an arc-extinguishing material, such as melamine, and a thermoplastic structural binding polymer, such as ethylene acrylic acid copolymer to achieve a combination of arc-extinguishing properties and improved structural characteristics, such as tensile strength, elongation, and environmental resistance to thermal cycling. The composition according to Syvertson includes an improved thermoplastic polymeric binder containing carboxylic acid moieties, such as ethylene acrylic acid, wherein the carboxylic acid moiety of the binder polymer is chemically bonded to an arc-extinguishing material, such as melamine, containing a carboxylic acid reactive group, such as amine, hydroxyl, epoxy, aziridine or thiol groups during structural molding of the arc-extinguishing composition under heat and pressure.
However, an arc-extinguishing composition according to Syvertson involves high material and fabrication costs to produce and further involves highly carbonizing carboxylic acid groups which in the fused state will likely form tracking conditions and create difficulties in quenching the arc.